Smart move by DOJ lawyers. The letter below informs Judiciary Chairman Jerry Nadler the previous report content was provided without assertion of executive privilege; however, if Nadler follows-through with impeachment plan, executive privilege is now enforced and the totality of the report is withdrawn from congress.
The only thing AG Barr was statutorily required to provide was the four-page summary letter he already presented. Asking President Trump to throw the executive privilege blanket over the full work product effectively shields it from congressional review; AND simultaneously blocks congress from proceeding with any impeachment action.
The assertion of executive privilege effectively removes any lack of production action (ie. the impeachment threat) by Chairman Jerry Nadler.
Essentially AG Barr et al is going old school with classic separation of power. POTUS Trump built a bridge from executive to legislative branch with voluntary production; however, if Nadler wants to be a resistance member then the executive branch will pull back and the customary three-branch separation of power rules will be the standard form of engagement.